This invention relates to cathode ray tubes and more particularly to apparatus and a method for manufacturing cathode ray tube screens.
In the manufacture of screens for cathode ray tubes and especially in the utilization of a light to expose a pattern of photosensitive material in the fabrication of color cathode ray tubes, it has been a common practice to provide a primary light source which is used to provide a secondary or point source of light. Such an optical system ordinarily utilizes a "light pipe" or quartz rod to effect light transmission. However, due to internal reflections, it is not unexpected that a large portion of the intensity of the primary light source is lost in such optical system processing.
Also, it has been conventional practice in the manufacture of color cathode ray tube viewing screens to employ a faceplate for the cathode ray tube with a layer of photosensitive material and color emitting phosphors on the inner surface of the faceplate. An apertured mask is affixed to the faceplate and spaced from the layer of photosensitive material and color emitting phosphors.
The faceplate is placed on a screen exposure apparatus, referred to as a "lighthouse", and the light source is disposed within the "lighthouse" and spaced from the faceplate. A correction lens for directing the light rays from the light source to the faceplate as well as a light inhibiter or "shader" are disposed intermediate the light source and the faceplate and serve to control light ray impingement of the faceplate. Moreover, a shutter is disposed intermediate the light source and the lens and "shader" and utilized to initiate or interrupt light beams emanating from the light source to either permit or inhibit impingement of the faceplate by the light beams.
In this manner actinic energy from the light source is directed by the lens and "shader" and caused to pass through the apertures of the mask to impinge the photosensitive layer of the faceplate. Thereupon, specific portions of the photosensitive layer are hardened and the unexposed and unhardened portions removed as by washing. Also, the technique is repeated by altering the positional location of the light source with respect to the faceplate to provide matrix windows or phosphor-adhering portions formed for impingement by electron beams of a cathode ray tube.
Although the above-described manufacturing technique has been and still is utilized to provide patterned screens for cathode ray tubes, it has been found that certain difficulties are encountered. For example, it has been found that optic systems utilizing primary and secondary light sources or a "light pipe" require something in the neighborhood of a one kilowatt primary light source due to the attenuation of the "light pipe". As a result, an elaborate cooling system is required to keep the lamp operational and such systems are both expensive and cumbersome.
Also, cathode ray tube screens for so-called "dot" or delta screens required a point source of light to effect correct exposure. To achieve such a point source, the so-called "light pipe" technique was a necessity. However, fabrication of an inline type of color cathode ray tube does not require a point source of light whereupon a direct viewing light source of greatly reduced energy requirements is possible and desirable. Moreover, such a direct viewing light source is preferably movable to provide proper effective source length to insure straight matrix windows or phosphor lines.
Furthermore, exposure time for the indirect or "light pipe" type of system was frequently counted in terms of minutes whereas a direct viewing system, having an optic system with up to ten magnitudes of increased light capability, operates with an exposure time counted in terms of seconds. As a result, prior techniques whereby a shutter was indexed into and indexed back out of the light source path are no longer satisfactory. In other words, the shortness of exposure time requires a precision type of shutter operation in order to effect the desired uniformity of exposure throughout the entire faceplate.